The present invention relates to an actuator for use in a vehicle-door locking mechanism, which includes an automatic bidirectional-returning mechanism using a single return coil spring as a home returning spring for allowing a manual operation.
As such an actuator, there is a conventional one shown in FIGS. 5 and 6A to 6C. FIG. 5 is a plan view of the constitution of a major part of the prior art actuator. In this figure, reference numeral 51 denotes a driving motor, 52 shows a small gear such as a helical pinion attached to a shaft of the driving motor, and 53 indicates a large gear such as a helical worm gear engaged with the small gear 52. Furthermore, reference numeral 54 shows a lead screw serving as a main shaft fixed to the large gear 53 so as to penetrate the center thereof and having a screw section 54a on the circumference thereof, 55 indicates a nut member fitted on the lead screw 54 and moved along the axis of the screw 54 in accordance with the rotation of the screw 54, and 56 denotes a lever turned on its axis within the range of a given angle in accordance with the movement of the nut member 55. Reference numeral 57 denotes an output shaft provided coaxially with the axis of the lever 56 and numeral 58 indicate s an output arm for transmitting the rotation force of the output shaft 57 to a door locking mechanism (not shown).
An automatic bidirectional-returning mechanism 60 for returning the lead screw 54 to its home position (initial position) is mounted on an elongated end portion of the lead screw 54 which penetrates the large gear 53 toward the right side of FIG. 5.
FIG. 6A is a perspective view of the constitution of the automatic bidirectional-returning mechanism 60. Referring to FIG. 6A, the mechanism 60 includes a bobbin 61 fixed coaxially to the elongated end portion of the lead screw 54. The bobbin 61 includes a cylindrical section (not shown) having a predetermined length and located on its axis, a pair of flanges 61a and 61b provided on both ends of the cylindrical section so as to be opposed to each other, and a strip-like operation member 61c so as to build a bridge between the flanges 61 and 61b.
A single home-returning coil spring 62 is wound around the bobbin 61. Both ends of the coil spring 62 are each bent like a letter "L" in the radial direction thereof, and these bent portions serve as engaging end portions 62a and 62b.
One engaging end portion 62a passes near one side of the operation member 61c of the bobbin 61 and its tip is brought into contact with one side of a stopper 64 at a given pressure. The other engagement end portion 62b passes near the other side of the operation member 61c of the bobbin 61 and its tip is brought into contact with the other side of the stopper 64 at a given pressure.
The stopper 64 is formed on a mounting base 63 of an actuator holding case integrally with the base 63 as one unit. The stopper 64 is formed of a rectangular projection in parallel with the axis of the coil spring 62.
The prior art actuator so constituted operates as follows. If the driving motor 51 rotates forward to lock the door of a vehicle, the small gear 52 rotates in the direction of arrow A in FIG. 5 and accordingly the large gear 53 rotates in the direction of arrow B. The nut member 55 thus moves relatively in the direction of arrow C. A projection 55a of the nut member 55 is then pressed on the left inner side of a fitting window 56a of the lever 56 in FIG. 5. The lever 56 therefore turns in the direction of arrow D1. As the lever 56 turns, the output arm 58 turns around its output axis 57 in the direction of arrow E1. If the output arm 58 turns by a distance corresponding to a stroke S1, the door locking mechanism (not shown) is locked.
When the large gear 53 and lead screw 54 start rotating in the direction of arrow B, the bobbin 61 of the mechanism 60, fixed to the lead screw 54, also starts rotating in the same direction. The operation member 61c thus causes the engaging end portion 62a of the coil spring 62 to be biased in the direction of arrow F1 in FIG. 6B. Since the other engagement end portion 62b of the coil spring 62 is engaged with the other side of the stopper 64, the coil spring 62 is compressed gradually according to the rotation of the operation member 61c. As indicated by the broken line in FIG. 6C, when the engaging end portion 62a biased by the operation member 61c reaches and contacts the other side of the stopper 64, the portion 62a cannot rotate any more.
In this state, the power of the driving motor 51 is cut off by means of, e.g., a limit switch and the motor 51 stops rotating accordingly. If the driving motor 51 stops, the decompression force of the compressed coil spring 62 is transmitted to the lead screw 54 through the bobbin 61 and also to the motor 51 through the small and large gears 52 and 53. The motor 51 and lead screw 54 thus rotate backward. The nut member 55 moves in a direction opposite to that of arrow C and returns to its initial position. When the engagement end portion 62a of the coil spring 62 returns to one side of the stopper 64, the above decompression force is lost. The nut member 55 is therefore returned to the initial position and stabilized.
The returning operation of the nut member 55 is performed independently within the range of the fitting window 56a of the lever 56 such that it does not contact the lever 56. The lever 56 thus remains stationary in which position a door locking operation is performed or in which position the lever 56 is rotated only through an angle .theta.1.
When the driving motor 51 rotates backward to unlock the vehicle door, the small gear 52, large gear 53 and lead screw 54 rotate in a direction opposite to the above direction, and the nut member 55 moves in a direction opposite to that of arrow C. The lever 56 thus turns in the direction of arrow D2, the output shaft 57 rotates in the same direction, and the output arm 58 turns in the direction of arrow E2. If the output arm 58 turns by a distance corresponding to a stroke S2, the door locking mechanism is unlocked.
The automatic bidirectional-returning mechanism 60 performs an operation opposite to the foregoing operation. More specifically, the engagement end portion 62b of the coil spring 62 is biased in the direction of arrow F2 in FIG. 6B such that the portion 62b is separated from the other side of the stopper 64 by means of the operation member 61c of the bobbin 61. When the engagement end portion 62b reaches and contacts one side of the stopper 64, the bias operation stops. In this time, a limit switch (not shown) operates to cut off the power of the driving motor 51 and stop its rotation.
In the prior art door locking actuator having the above constitution, the lead screw 54 can rotate only one rotation or less in either the forward or backward direction. Usually, the lead screw 54 can turn only .+-.0.88 turn. The operation end of the output arm 58 thus needs shifting by a required stroke S1=S2 (about 15 mm at the tip of the arm) in order to sufficiently operate the door locking mechanism and accordingly the lead angle .beta. of the lead screw 54 has to be considerably large. If the lead angle .beta. is increased, naturally, the driving force of the door locking mechanism is likely to lower to cause a malfunction.
The lead angle .beta. is obtained by the following equation: tan.beta.=L/2.pi. r, where L represents a lead (the distance by which the screw advances). Incidentally, the lead L of the conventional lead screw is 6.16 mm.
To achieve the above stroke S1=S2, the lead L of the lead screw 54 should be set to 8.1 mm or more. However, this causes the problem that the torque of the lead screw 54 is decreased extremely and a necessary amount of torque cannot be obtained.
To compensate for the inadequacy of torque, it is necessary to increase the axle ratio of a deceleration gear mechanism including the gears 52 and 53 for reducing the rotation speed of the driving motor 51 and transmitting it to the lead screw 54. If the axle ratio is increased, the torque inadequacy can be prevented but the rotation speed becomes low. Consequently, the door locking mechanism becomes difficult to operate at a prescribed rate (0.5 second or lower), thereby causing a drawback that the response speed of the lock or unlock operation of the door locking mechanism is low.